Physiology of Colon, Rectum, and Anus



Fig. 3.1
Rectal motor complex



Two types of change in rectal tone have been described:



  • Rapid-volume waves


  • Slow-volume waves

Rapid-volume waves last less than 2 min and are associated with increased luminal pressure. They are not affected by eating. Slow-volume waves last more than 2 min and are not associated with changes in intraluminal pressure, but their frequency increases after a meal. Slow waves may increase rectal sensation of luminal contents. Increased rectal tone during defecation may change the rectum from a capacious reservoir to a conduit.



3.2.5 Postprandial and Diurnal Changes


Colorectal tone and the frequency of both colonic mass contractions and haustral colonic contractions increase within a few minutes after a meal. The effect is more pronounced in the left than in the right colon, and it usually lasts 30–60 min. This gastrocolic response is mediated by sympathetic nerves and by the release of cholecystokinin and perhaps gastrin. The effect is to move contents over large distances of the colorectum, often resulting in defecation.

Sleep has a strong inhibitory effect on colonic mass contractions, haustral contractions, and colorectal tone. However, during rapid eye movement sleep and especially upon awakening, colonic tonic and phasic activity increases. The RMC is more frequent during sleep and may contribute to nocturnal continence.


3.2.6 Neural Control of Colorectal Motility


Colorectal motility is controlled by various factors:



  • Enteric nervous system (ENS)


  • Prevertebral sympathetic ganglia


  • Autonomic system within the brain stem and spinal cord


  • Higher cortical centers


  • Circulating hormones


  • Immune system


3.2.6.1 Enteric Nervous System


Enteric nerves within the intermuscular plexus (Auerbach’s plexus) mainly control colorectal motility, and those within the submucosal plexus (Meissner’s plexus) mainly control mucosal secretion and blood flow. Neurotransmitters found in the ENS can either stimulate (acetylcholine, serotonin, histamine, cholecystokinin, angiotensin, motilin, and gastrin) or inhibit (dopamine, noradrenalin, glucagon, vasoactive intestinal polypeptide, enkephalin, and somatostatin) motility. Receptors for histamine and serotonin have been classified into subgroups. Agonists and antagonists have been developed and may have a clinical role in the future.

The ENS generally consists of three types of neurons:



  • Sensory neurons


  • Interneurons


  • Motor neurons

Sensory neurons, specialized to detect mechanical stimuli, temperature, or chemical properties, interact through multiple interneurons with motor neurons to either stimulate or inhibit smooth muscle contraction. Interneurons also integrate stimuli from the ENS with the extrinsic nerve system and hormones. Reflexes within the ENS can thus be activated by both local and extrinsic stimuli. Thus efferent parasympathetic fibers within the vagal and splanchnic nerves can stimulate motility over large distances of the gastrointestinal tract.


3.2.6.2 Prevertebral Sympathetic Ganglia


Sympathetic nerve fibers and prevertebral sympathetic ganglia are considered the most important mediators of the gastrocolic response, which mediates colorectal phasic and tonic activity after a meal. Parasympathetic activity within the ENS depolarizes colorectal smooth muscle cells through the release of acetylcholine and stimulates colorectal motility. If parasympathetic innervation is lost, colorectal reflex activity is reduced. A clinically important example is severe defecation disorders caused by reduced left colonic and rectal reflex activity and tone after damage to the splanchnic nerves or spinal cord lesions of the conus medullaris or cauda equina.

Sympathetic activity causes hyperpolarization of colorectal smooth muscle cells, reducing colonic phasic activity and tone. The clinical effects of sympathetic denervation have not been studied in detail, but observational studies indicate that it has a minor effect on colorectal transport.


3.2.6.3 Autonomic System


Nonconscious sensory information is mediated through parasympathetic afferents in the vagal nerve or through the splanchnic nerves to the sacral spinal cord. Painful stimuli are conveyed through sympathetic afferents via a three-neuron chain from the colon to the brain: the cell body of the primary afferent is located in the dorsal root ganglia of the spinal cord. This synapses with dorsal horn neurons and conveys information through the spinothalamic or spinoreticular tracts to the thalamus and reticular formation. From there, a third neuron connects to higher sensory centers such as the anterior cingulate cortex.

The colon and rectum are insensitive to most stimuli; however, they are very sensitive to stretching. The subjective experience of rectal sensation is a feeling of rectal fullness and an urge to defecate. By contrast, colonic distension produces pain and colic. The location of rectal stretch receptors is controversial. The rectal mucosa contains no specific receptor type, which probably explains the poor discriminatory quality of rectal sensation.


3.2.6.4 Higher Cortical Centers


Higher brain centers that influence colonic motility include the frontal regions of the cerebral cortex, the stria terminalis, the amygdala, and the hypothalamus. The effects on colorectal motility are mainly inhibitory; thus a loss of supraspinal control of the sacral reflex center may cause increased left colonic and rectal reflex activity and tone (Fig. 3.2).

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Fig. 3.2
Control of colorectal motility. White arrows: the enteric nervous system; solid lines: parasympathetic innervation; broken lines: sympathetic innervation


3.2.6.5 Hormonal and Immune System Control


Thyroid hormone stimulates colorectal motility and epinephrine reduces it. The unique ability of the immune system to recognize specific antigens makes immunoneuronal integration important for bowel function. Once the immune system within the bowel wall becomes sensitized to specific antigens, a second exposure to that antigen causes mast cells to release histamine and other messengers. Histamine acts on intestinal H2 receptors, stimulating electrolyte, water, and mucus secretion, and promotes strong contractions, called “power propulsion,” spanning large distances within the bowel. Consequently, potentially harmful antigens are quickly cleared from the lumen.



3.3 Colorectal Transit Time


Total and segmental colorectal transit times show great individual variation. Healthy asymptomatic subjects may have total colorectal transit times of up to 4 days. Left colonic and rectal transit time is usually longer than right colonic transit time. In healthy subjects, stool frequency and consistency probably correlate better with rectosigmoid transit time than with total colonic transit time. However, stool weight per day correlates with colonic transit time.

Stool weight in healthy people consuming a normal diet in Europe or North America is usually between 100 and 150 g/day. In rural Uganda it is up to 500 g/day. Dietary fibers, mainly bran, that do not undergo anaerobic bacterial fermentation retain water within stools. Accordingly, bran increases stool weight and reduces colonic transit time in most individuals. It is, however, important that extra fiber does not reduce colonic transit times in women with severe idiopathic constipation; it may even further prolong transit times in patients with severely prolonged colonic transit times caused by spinal cord lesions.


3.4 Anorectal Physiology


The main functions of the rectum and anal canal are:



  • To maintain fecal continence


  • To allow defecation at an appropriate time and place

The following factors are important in maintaining anal continence:



  • Internal anal sphincter muscle (IAS)


  • External anal sphincter muscle (EAS)


  • Puborectalis muscle


  • Rectal compliance


  • Anorectal sensitivity


  • Anorectal motility


3.4.1 Internal Anal Sphincter


The IAS is a continuation of the circular muscle layer of the rectum and consists of smooth muscle cells. Its main function is to contribute to the anal resting pressure. Anal resting pressure is extremely variable between individuals and tends to decrease with age and parity. The resting pressure undulates in a slow-wave pattern of low amplitude and frequency. An ultra-slow-wave pattern of greater amplitude may also be present. Their physiological significance is unknown.

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Oct 30, 2017 | Posted by in ABDOMINAL MEDICINE | Comments Off on Physiology of Colon, Rectum, and Anus
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